RFC 6185

Internet Engineering Task Force (IETF) T. Kristensen
Request for Comments: 6185 P. Luthi
Category: Standards Track TANDBERG
ISSN: 2070-1721 May 2011 RTP Payload Format for
H.264 Reduced-Complexity Decoding Operation (RCDO) Video
Abstract
This document describes an RTP payload format for the Reduced-
Complexity Decoding Operation (RCDO) for H.264 Baseline profile
bitstreams, as specified in ITU-T Recommendation H.241. RCDO reduces
the decoding cost and resource consumption of the video processing.
The RCDO RTP payload format is based on the H.264 RTP payload format.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6185.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [4].
3. Media Format Background
The Reduced-Complexity Decoding Operation (RCDO) for H.264 Baseline
profile bitstreams is specified in Annex B of H.241 [3]. RCDO is
specified as a separate H.264 mode and is distinct from any profile
defined in H.264. An RCDO bitstream obeys all the constraints of the
Baseline profile.
The media format is based on the H.264 RTP payload format as
specified in RFC 6184 [1]. Therefore, RFC 6184 constitutes the basis
for this document and is referred to several times.
In order to signal H.264 additional modes, Table 8-13 of H.241 [3]
specifies an AdditionalModesSupported parameter. Currently, the only
additional mode defined is RCDO.
Informative note: Other additional modes may be defined in the
future. H.264 additional modes may or may not be distinct from
the profiles in H.264.
A separate media subtype, named H264-RCDO, is defined to ensure
backward compatibility with deployed implementations of H.264.
4. Payload Format
The payload format defined in Section 5 of RFC 6184 [1] SHALL be
used. This includes the RTP header usage and the payload format in
RFC 6184. Examples of typical RTP packets can be found in RFC 6184.
5. Congestion Control Considerations
Congestion control for RTP SHALL be used in accordance with RFC 3550
[6] and with any applicable RTP profile, e.g., RFC 3551 [7]. If
best-effort service is being used, users of this payload format SHALL
monitor packet loss to ensure that the packet loss rate is within
acceptable parameters.
6. Payload Format Parameters
This RTP payload format is identified using the H264-RCDO media
subtype, which is registered in accordance with RFC 4855 [10], and
using the template of RFC 4288 [13].

6.1. Media Type Definition
Informative note: The media subtype definition for H264-RCDO is
based on the definition of the H264 media subtype as specified in
Section 8.1 of RFC 6184 [1]. Except for the profile-level-id
parameter, for which new semantics are specified below, the
optional parameters are copied from RFC 6184 [1] in order to
provide a complete, self-contained media subtype registration to
IANA. The references are updated to match the numbering used in
this document.
The media subtype for RCDO for H.264 has been allocated from the IETF
tree.
Type name: video
Subtype name: H264-RCDO
Required parameters:
rate: Indicates the RTP timestamp clock rate. The rate value MUST
be 90000.
Optional parameters:
profile-level-id: A base16 RFC 4648 [9] (hexadecimal) representation
of the following three bytes in the sequence parameter set NAL
unit is specified in H.264 [2]: 1) profile_idc, 2) a byte herein
referred to as profile-iop, composed of the values of
constraint_set0_flag, constraint_set1_flag, constraint_set2_flag,
constraint_set3_flag, constraint_set4_flag, constraint_set5_flag,
and reserved_zero_2bits in bit-significance order, starting from
the most-significant bit, and 3) level_idc. Note that
reserved_zero_2bits is required to be equal to 0 in H.264 [2], but
other values for it may be specified in the future by ITU-T or
ISO/IEC.
The profile-level-id parameter indicates the default sub-profile
(i.e., the subset of coding tools that may have been used to
generate the stream or that the receiver supports) and the default
level of the stream or the receiver supports.
RCDO is distinct from any profile; this implies that the profile
value 0 (no profile) and the profile_idc byte of the profile-
level-id parameter are equal to 0. An RCDO bitstream MUST obey
all the constraints of the Baseline profile. Therefore, only
constraint_set0_flag is equal to 1 in the profile-iop part of the
profile-level-id parameter; the remaining bits are set to 0.

If the profile-level-id parameter is used to indicate properties
of a NAL unit stream, it indicates that, to decode the stream, the
minimum subset of coding tools a decoder has to support is the
default sub-profile, and the lowest level the decoder has to
support is the default level.
If the profile-level-id parameter is used for capability exchange
or session setup, it indicates the subset of coding tools, which
is equal to the default sub-profile, that the codec supports for
both receiving and sending. If max-recv-level is not present, the
default level from profile-level-id indicates the highest level
the codec wishes to support. If max-recv-level is present, it
indicates the highest level the codec supports for receiving. For
either receiving or sending, all levels that are lower than the
highest level supported MUST also be supported.
For example, if a codec supports level 1.3, the profile-level-id
becomes 00800d, in which 00 indicates the "no profile" value, 80
indicates the constraints of the Baseline profile, and 0d
indicates level 1.3. When level 2.1 is supported, the profile-
level-id becomes 008015.
If no profile-level-id is present, level 1 (i.e., equivalent to
profile-level-id 00800a) MUST be implied.
Informative note: The definitions of the remaining optional
parameters below are copied verbatim from Section 8.1 of RFC
6184 [1]. Only the references are updated to match the
numbering used in this document.
max-recv-level: This parameter MAY be used to indicate the highest
level a receiver supports when the highest level is higher than
the default level (the level indicated by profile-level-id). The
value of max-recv-level is a base16 (hexadecimal) representation
of the two bytes after the syntax element profile_idc in the
sequence parameter set NAL unit specified in H.264 [2]: profile-
iop (as defined above) and level_idc. If the level_idc byte of
max-recv-level is equal to 11 and bit 4 of the profile-iop byte of
max-recv-level is equal to 1 or if the level_idc byte of max-recv-
level is equal to 9 and bit 4 of the profile-iop byte of max-recv-
level is equal to 0, the highest level the receiver supports is
Level 1b. Otherwise, the highest level the receiver supports is
equal to the level_idc byte of max-recv-level divided by 10.
max-recv-level MUST NOT be present if the highest level the
receiver supports is not higher than the default level.

max-mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br: These
parameters MAY be used to signal the capabilities of a receiver
implementation. These parameters MUST NOT be used for any other
purpose. The highest level conveyed in the value of the profile-
level-id parameter or the max-recv-level parameter MUST be such
that the receiver is fully capable of supporting. max-mbps, max-
smbps, max-fs, max-cpb, max-dpb, and max-br MAY be used to
indicate capabilities of the receiver that extend the required
capabilities of the signaled highest level, as specified below.
When more than one parameter from the set (max-mbps, max-smbps,
max-fs, max-cpb, max-dpb, max-br) is present, the receiver MUST
support all signaled capabilities simultaneously. For example, if
both max-mbps and max-br are present, the signaled highest level
with the extension of both the frame rate and bitrate is
supported. That is, the receiver is able to decode NAL unit
streams in which the macroblock processing rate is up to max-mbps
(inclusive), the bitrate is up to max-br (inclusive), the coded
picture buffer size is derived as specified in the semantics of
the max-br parameter below, and the other properties comply with
the highest level specified in the value of the profile-level-id
parameter or the max-recv-level parameter.
If a receiver can support all the properties of Level A, the
highest level specified in the value of the profile-level-id
parameter or the max-recv-level parameter MUST be Level A (i.e.,
MUST NOT be lower than Level A). In other words, a receiver MUST
NOT signal values of max-mbps, max-fs, max-cpb, max-dpb, and
max-br that taken together meet the requirements of a higher level
compared to the highest level specified in the value of the
profile-level-id parameter or the max-recv-level parameter.
Informative note: When the OPTIONAL media type parameters are
used to signal the properties of a NAL unit stream, max-mbps,
max-smbps, max-fs, max-cpb, max-dpb, and max-br are not
present, and the value of profile-level-id must always be such
that the NAL unit stream complies fully with the specified
profile and level.
max-mbps: The value of max-mbps is an integer indicating the maximum
macroblock processing rate in units of macroblocks per second.
The max-mbps parameter signals that the receiver is capable of
decoding video at a higher rate than is required by the signaled
highest level conveyed in the value of the profile-level-id
parameter or the max-recv-level parameter. When max-mbps is
signaled, the receiver MUST be able to decode NAL unit streams
that conform to the signaled highest level, with the exception
that the MaxMBPS value in Table A-1 of H.264 [2] for the signaled

highest level is replaced with the value of max-mbps. The value
of max-mbps MUST be greater than or equal to the value of MaxMBPS
given in Table A-1 of H.264 [2] for the highest level. Senders
MAY use this knowledge to send pictures of a given size at a
higher picture rate than is indicated in the signaled highest
level.
max-smbps: The value of max-smbps is an integer indicating the
maximum static macroblock processing rate in units of static
macroblocks per second, under the hypothetical assumption that all
macroblocks are static macroblocks. When max-smbps is signaled,
the MaxMBPS value in Table A-1 of H.264 [2] should be replaced
with the result of the following computation:
o If the parameter max-mbps is signaled, set a variable
MaxMacroblocksPerSecond to the value of max-mbps. Otherwise,
set MaxMacroblocksPerSecond equal to the value of MaxMBPS in
Table A-1 of H.264 [2] for the signaled highest level conveyed
in the value of the profile-level-id parameter or the
max-recv-level parameter.
o Set a variable P_non-static to the proportion of non-static
macroblocks in picture n.
o Set a variable P_static to the proportion of static macroblocks
in picture n.
o The value of MaxMBPS in Table A-1 of H.264 [2] should be
considered by the encoder to be equal to:
MaxMacroblocksPerSecond * max-smbps / (P_non-static * max-smbps
+ P_static * MaxMacroblocksPerSecond)
The encoder should recompute this value for each picture. The
value of max-smbps MUST be greater than or equal to the value of
MaxMBPS given explicitly as the value of the max-mbps parameter or
implicitly in Table A-1 of H.264 [2] for the signaled highest
level. Senders MAY use this knowledge to send pictures of a given
size at a higher picture rate than is indicated in the signaled
highest level.
max-fs: The value of max-fs is an integer indicating the maximum
frame size in units of macroblocks. The max-fs parameter signals
that the receiver is capable of decoding larger picture sizes than
are required by the signaled highest level conveyed in the value
of the profile-level-id parameter or the max-recv-level parameter.
When max-fs is signaled, the receiver MUST be able to decode NAL
unit streams that conform to the signaled highest level, with the

exception that the MaxFS value in Table A-1 of H.264 [2] for the
signaled highest level is replaced with the value of max-fs. The
value of max-fs MUST be greater than or equal to the value of
MaxFS given in Table A-1 of H.264 [2] for the highest level.
Senders MAY use this knowledge to send larger pictures at a
proportionally lower frame rate than is indicated in the signaled
highest level.
max-cpb: The value of max-cpb is an integer indicating the maximum
coded picture buffer size in units of 1000 bits for the VCL HRD
parameters and in units of 1200 bits for the NAL HRD parameters.
Note that this parameter does not use units of cpbBrVclFactor and
cpbBrNALFactor (see Table A-1 of H.264 [2]). The max-cpb
parameter signals that the receiver has more memory than the
minimum amount of coded picture buffer memory required by the
signaled highest level conveyed in the value of the
profile-level-id parameter or the max-recv-level parameter. When
max-cpb is signaled, the receiver MUST be able to decode NAL unit
streams that conform to the signaled highest level, with the
exception that the MaxCPB value in Table A-1 of H.264 [2] for the
signaled highest level is replaced with the value of max-cpb
(after taking cpbBrVclFactor and cpbBrNALFactor into consideration
when needed). The value of max-cpb (after taking cpbBrVclFactor
and cpbBrNALFactor into consideration when needed) MUST be greater
than or equal to the value of MaxCPB given in Table A-1 of H.264
[2] for the highest level. Senders MAY use this knowledge to
construct coded video streams with greater variation of bitrate
than can be achieved with the MaxCPB value in Table A-1 of H.264
[2].
Informative note: The coded picture buffer is used in the
hypothetical reference decoder (Annex C of H.264). The use of
the hypothetical reference decoder is recommended in H.264
encoders to verify that the produced bitstream conforms to the
standard and to control the output bitrate. Thus, the coded
picture buffer is conceptually independent of any other
potential buffers in the receiver, including de-interleaving
and de-jitter buffers. The coded picture buffer need not be
implemented in decoders as specified in Annex C of H.264, but
rather standard-compliant decoders can have any buffering
arrangements provided that they can decode standard-compliant
bitstreams. Thus, in practice, the input buffer for a video
decoder can be integrated with de-interleaving and de-jitter
buffers of the receiver.

max-dpb: The value of max-dpb is an integer indicating the maximum
decoded picture buffer size in units of 8/3 macroblocks. The max-
dpb parameter signals that the receiver has more memory than the
minimum amount of decoded picture buffer memory required by the
signaled highest level conveyed in the value of the
profile-level-id parameter or the max-recv-level parameter. When
max-dpb is signaled, the receiver MUST be able to decode NAL unit
streams that conform to the signaled highest level, with the
exception that the MaxDpbMbs value in Table A-1 of H.264 [2] for
the signaled highest level is replaced with the value of max-dpb *
3 / 8. Consequently, a receiver that signals max-dpb MUST be
capable of storing the following number of decoded frames,
complementary field pairs, and non-paired fields in its decoded
picture buffer:
Min(max-dpb * 3 / 8 / ( PicWidthInMbs * FrameHeightInMbs), 16)
Wherein PicWidthInMbs and FrameHeightInMbs are defined in H.264
[2].
The value of max-dpb MUST be greater than or equal to the value of
MaxDpbMbs * 3 / 8, wherein the value of MaxDpbMbs is given in
Table A-1 of H.264 [2] for the highest level. Senders MAY use
this knowledge to construct coded video streams with improved
compression.
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. The
decoded picture buffer stores reconstructed samples. There is
no relationship between the size of the decoded picture buffer
and the buffers used in RTP, especially de-interleaving and
de-jitter buffers.
Informative note: In RFC 3984, which is obsoleted by RFC 6184,
the unit of this parameter was 1024 bytes. The unit has been
changed to 8/3 macroblocks in this document. The reason for
this change was due to the changes from the 2003 version of the
H.264 specification referenced by RFC 3984 to the 2010 version
of the H.264 specification referenced by this document,
particularly the changes to Table A-1 in the H.264
specification due to addition of color formats and bit depths
not supported earlier. The changed semantics of this parameter
keeps backward compatibility to RFC 3984 and supports all
profiles defined in the 2010 version of the H.264
specification.

max-br: The value of max-br is an integer indicating the maximum
video bitrate in units of 1000 bits per second for the VCL HRD
parameters and in units of 1200 bits per second for the NAL HRD
parameters. Note that this parameter does not use units of
cpbBrVclFactor and cpbBrNALFactor (see Table A-1 of H.264 [2]).
The max-br parameter signals that the video decoder of the
receiver is capable of decoding video at a higher bitrate than is
required by the signaled highest level conveyed in the value of
the profile-level-id parameter or the max-recv-level parameter.
When max-br is signaled, the video codec of the receiver MUST be
able to decode NAL unit streams that conform to the signaled
highest level, with the following exceptions in the limits
specified by the highest level:
o The value of max-br (after taking cpbBrVclFactor and
cpbBrNALFactor into consideration when needed) replaces the
MaxBR value in Table A-1 of H.264 [2] for the highest level.
o When the max-cpb parameter is not present, the result of the
following formula replaces the value of MaxCPB in Table A-1 of
H.264 [2]: (MaxCPB of the signaled level) * max-br / (MaxBR of
the signaled highest level).
For example, if a receiver signals capability for Main profile
Level 1.2 with max-br equal to 1550, this indicates a maximum
video bitrate of 1550 kbits/sec for VCL HRD parameters, a maximum
video bitrate of 1860 kbits/sec for NAL HRD parameters, and a CPB
size of 4036458 bits (1550000 / 384000 * 1000 * 1000).
The value of max-br (after taking cpbBrVclFactor and
cpbBrNALFactor into consideration when needed) MUST be greater
than or equal to the value MaxBR given in Table A-1 of H.264 [2]
for the signaled highest level.
Senders MAY use this knowledge to send higher bitrate video as
allowed in the level definition of Annex A of H.264 to achieve
improved video quality.
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. The
assumption that the network is capable of handling such
bitrates at any given time cannot be made from the value of
this parameter. In particular, no conclusion can be drawn that
the signaled bitrate is possible under congestion control
constraints.

redundant-pic-cap: This parameter signals the capabilities of a
receiver implementation. When equal to 0, the parameter indicates
that the receiver makes no attempt to use redundant coded pictures
to correct incorrectly decoded primary coded pictures. When equal
to 0, the receiver is not capable of using redundant slices;
therefore, a sender SHOULD avoid sending redundant slices to save
bandwidth. When equal to 1, the receiver is capable of decoding
any such redundant slice that covers a corrupted area in a primary
decoded picture (at least partly), and therefore a sender MAY send
redundant slices. When the parameter is not present, a value of 0
MUST be used for redundant-pic-cap. When present, the value of
redundant-pic-cap MUST be either 0 or 1.
When the profile-level-id parameter is present in the same
signaling as the redundant-pic-cap parameter and the profile
indicated in profile-level-id is such that it disallows the use of
redundant coded pictures (e.g., Main profile), the value of
redundant-pic-cap MUST be equal to 0. When a receiver indicates
redundant-pic-cap equal to 0, the received stream SHOULD NOT
contain redundant coded pictures.
Informative note: Even if redundant-pic-cap is equal to 0, the
decoder is able to ignore redundant codec pictures provided
that the decoder supports a profile (Baseline, Extended) in
which redundant coded pictures are allowed.
Informative note: Even if redundant-pic-cap is equal to 1, the
receiver may also choose other error concealment strategies to
replace or complement decoding of redundant slices.
sprop-parameter-sets: This parameter MAY be used to convey any
sequence and picture parameter set NAL units (herein referred to
as the initial parameter set NAL units) that can be placed in the
NAL unit stream to precede any other NAL units in decoding order.
The parameter MUST NOT be used to indicate codec capability in any
capability exchange procedure. The value of the parameter is a
comma-separated (',') list of base64 RFC 4648 [9] representations
of parameter set NAL units as specified in Sections 7.3.2.1 and
7.3.2.2 of H.264 [2]. Note that the number of bytes in a
parameter set NAL unit is typically less than 10, but a picture
parameter set NAL unit can contain several hundred bytes.
Informative note: When several payload types are offered in the
SDP Offer/Answer model, each with its own sprop-parameter-sets
parameter, the receiver cannot assume that those parameter sets
do not use conflicting storage locations (i.e., identical
values of parameter set identifiers). Therefore, a receiver

should buffer all sprop-parameter-sets and make them available
to the decoder instance that decodes a certain payload type.
The sprop-parameter-sets parameter MUST only contain parameter
sets that are conforming to the profile-level-id, i.e., the subset
of coding tools indicated by any of the parameter sets MUST be
equal to the default sub-profile, and the level indicated by any
of the parameter sets MUST be equal to the default level.
sprop-level-parameter-sets: This parameter MAY be used to convey any
sequence and picture parameter set NAL units (herein referred to
as the initial parameter set NAL units) that can be placed in the
NAL unit stream to precede any other NAL units in decoding order
and that are associated with one or more levels different than the
default level. The parameter MUST NOT be used to indicate codec
capability in any capability exchange procedure.
The sprop-level-parameter-sets parameter contains parameter sets
for one or more levels that are different than the default level.
All parameter sets associated with one level are clustered and
prefixed with a three-byte field that has the same syntax as
profile-level-id. This enables the receiver to install the
parameter sets for one level and discard the rest. The three-byte
field is named PLId, and all parameter sets associated with one
level are named PSL, which has the same syntax as sprop-parameter-
sets. Parameter sets for each level are represented in the form
of PLId:PSL, i.e., PLId followed by a colon (':') and the base64
RFC 4648 [9] representation of the initial parameter set NAL units
for the level. Each pair of PLId:PSLs is also separated by a
colon. Note that a PSL can contain multiple parameter sets for
that level, separated with commas (',').
The subset of coding tools indicated by each PLId field MUST be
equal to the default sub-profile, and the level indicated by each
PLId field MUST be different than the default level. All sequence
parameter sets contained in each PSL MUST have the three bytes
from profile_idc to level_idc, inclusive, equal to the preceding
PLId.
Informative note: This parameter allows for efficient level
downgrade or upgrade in SDP Offer/Answer and out-of-band
transport of parameter sets simultaneously.
use-level-src-parameter-sets: This parameter MAY be used to indicate
a receiver capability. The value MAY be equal to either 0 or 1.
When the parameter is not present, the value MUST be inferred to
be equal to 0. The value 0 indicates that the receiver does not
understand the sprop-level-parameter-sets parameter, does not

understand the "fmtp" source attribute as specified in Section 6.3
of RFC 5576 [14], will ignore sprop-level-parameter-sets when
present, and will ignore sprop-parameter-sets when conveyed using
the "fmtp" source attribute. The value 1 indicates that the
receiver understands the sprop-level-parameter-sets parameter,
understands the "fmtp" source attribute as specified in Section
6.3 of RFC 5576 [14], and is capable of using parameter sets
contained in the sprop-level-parameter-sets or contained in the
sprop-parameter-sets that is conveyed using the "fmtp" source
attribute.
Informative note: An RFC 3984 receiver does not understand
sprop-level-parameter-sets, use-level-src-parameter-sets, or
the "fmtp" source attribute as specified in Section 6.3 of RFC
5576 [14]. Therefore, during SDP Offer/Answer, an RFC 3984
receiver as the answerer will simply ignore sprop-level-
parameter-sets when present in an offer and sprop-parameter-
sets conveyed using the "fmtp" source attribute, as specified
in Section 6.3 of RFC 5576 [14]. Assume that the offered
payload type was accepted at a level lower than the default
level. If the offered payload type included sprop-level-
parameter-sets or included sprop-parameter-sets conveyed using
the "fmtp" source attribute and if the offerer sees that the
answerer has not included use-level-src-parameter-sets equal to
1 in the answer, the offerer knows that in-band transport of
parameter sets is needed.
in-band-parameter-sets: This parameter MAY be used to indicate a
receiver capability. The value MAY be equal to either 0 or 1.
The value 1 indicates that the receiver discards out-of-band
parameter sets in sprop-parameter-sets and sprop-level-parameter-
sets; therefore, the sender MUST transmit all parameter sets in-
band. The value 0 indicates that the receiver utilizes out-of-
band parameter sets included in sprop-parameter-sets and/or sprop-
level-parameter-sets. However, in this case, the sender MAY still
choose to send parameter sets in-band. When in-band-parameter-
sets is equal to 1, use-level-src-parameter-sets MUST NOT be
present or MUST be equal to 0. When the parameter is not present,
this receiver capability is not specified, and therefore the
sender MAY send out-of-band parameter sets only, it MAY send in-
band-parameter-sets only, or it MAY send both.
level-asymmetry-allowed: This parameter MAY be used in SDP Offer/
Answer to indicate whether level asymmetry, i.e., sending media
encoded at a different level in the offerer-to-answerer direction
than the level in the answerer-to-offerer direction, is allowed.
The value MAY be equal to either 0 or 1. When the parameter is
not present, the value MUST be inferred to be equal to 0. The

value 1 in both the offer and the answer indicates that level
asymmetry is allowed. The value of 0 in either the offer or the
answer indicates that level asymmetry is not allowed.
If level-asymmetry-allowed is equal to 0 (or not present) in
either the offer or the answer, level asymmetry is not allowed.
In this case, the level to use in the direction from the offerer
to the answerer MUST be the same as the level to use in the
opposite direction.
packetization-mode: This parameter signals the properties of an RTP
payload type or the capabilities of a receiver implementation.
Only a single configuration point can be indicated; thus, when
capabilities to support more than one packetization-mode are
declared, multiple configuration points (RTP payload types) must
be used.
When the value of packetization-mode is equal to 0 or
packetization-mode is not present, the single NAL mode MUST be
used. This mode is in use in standards using ITU-T Recommendation
H.241 [3] (see Section 12.1). When the value of packetization-
mode is equal to 1, the non-interleaved mode MUST be used. When
the value of packetization-mode is equal to 2, the interleaved
mode MUST be used. The value of packetization-mode MUST be an
integer in the range of 0 to 2, inclusive.
sprop-interleaving-depth: This parameter MUST NOT be present when
packetization-mode is not present or the value of packetization-
mode is equal to 0 or 1. This parameter MUST be present when the
value of packetization-mode is equal to 2.
This parameter signals the properties of an RTP packet stream. It
specifies the maximum number of VCL NAL units that precede any VCL
NAL unit in the RTP packet stream in transmission order and that
follow the VCL NAL unit in decoding order. Consequently, it is
guaranteed that receivers can reconstruct NAL unit decoding order
when the buffer size for NAL unit decoding order recovery is at
least the value of sprop-interleaving-depth + 1 in terms of VCL
NAL units.
The value of sprop-interleaving-depth MUST be an integer in the
range of 0 to 32767, inclusive.
sprop-deint-buf-req: This parameter MUST NOT be present when
packetization-mode is not present or the value of packetization-
mode is equal to 0 or 1. It MUST be present when the value of
packetization-mode is equal to 2.

sprop-deint-buf-req signals the required size of the
de-interleaving buffer for the RTP packet stream. The value of
the parameter MUST be greater than or equal to the maximum buffer
occupancy (in units of bytes) required in such a de-interleaving
buffer that is specified in Section 7.2 of RFC 6184 [1]. It is
guaranteed that receivers can perform the de-interleaving of
interleaved NAL units into NAL unit decoding order, when the
de-interleaving buffer size is at least the value of
sprop-deint-buf-req in terms of bytes.
The value of sprop-deint-buf-req MUST be an integer in the range
of 0 to 4294967295, inclusive.
Informative note: sprop-deint-buf-req indicates the required
size of the de-interleaving buffer only. When network jitter
can occur, an appropriately sized jitter buffer has to be
provisioned for as well.
deint-buf-cap: This parameter signals the capabilities of a receiver
implementation and indicates the amount of de-interleaving buffer
space in units of bytes that the receiver has available for
reconstructing the NAL unit decoding order. A receiver is able to
handle any stream for which the value of the sprop-deint-buf-req
parameter is smaller than or equal to this parameter.
If the parameter is not present, then a value of 0 MUST be used
for deint-buf-cap. The value of deint-buf-cap MUST be an integer
in the range of 0 to 4294967295, inclusive.
Informative note: deint-buf-cap indicates the maximum possible
size of the de-interleaving buffer of the receiver only. When
network jitter can occur, an appropriately sized jitter buffer
has to be provisioned for as well.
sprop-init-buf-time: This parameter MAY be used to signal the
properties of an RTP packet stream. The parameter MUST NOT be
present if the value of packetization-mode is equal to 0 or 1.
The parameter signals the initial buffering time that a receiver
MUST wait before starting decoding to recover the NAL unit
decoding order from the transmission order. The parameter is the
maximum value of (decoding time of the NAL unit - transmission
time of a NAL unit), assuming reliable and instantaneous
transmission, the same timeline for transmission and decoding, and
commencement of decoding when the first packet arrives.
An example of specifying the value of sprop-init-buf-time follows.
A NAL unit stream is sent in the following interleaved order, in

which the value corresponds to the decoding time and the
transmission order is from left to right:
0 2 1 3 5 4 6 8 7 ...
Assuming a steady transmission rate of NAL units, the transmission
times are:
0 1 2 3 4 5 6 7 8 ...
Subtracting the decoding time from the transmission time column-
wise results in the following series:
0 -1 1 0 -1 1 0 -1 1 ...
Thus, in terms of intervals of NAL unit transmission times, the
value of sprop-init-buf-time in this example is 1. The parameter
is coded as a non-negative base10 integer representation in clock
ticks of a 90-kHz clock. If the parameter is not present, then no
initial buffering time value is defined. Otherwise, the value of
sprop-init-buf-time MUST be an integer in the range of 0 to
4294967295, inclusive.
In addition to the signaled sprop-init-buf-time, receivers SHOULD
take into account the transmission delay jitter buffering,
including buffering for the delay jitter caused by mixers,
translators, gateways, proxies, traffic-shapers, and other network
elements.
sprop-max-don-diff: This parameter MAY be used to signal the
properties of an RTP packet stream. It MUST NOT be used to signal
transmitter, receiver, or codec capabilities. The parameter MUST
NOT be present if the value of packetization-mode is equal to 0 or
1. sprop-max-don-diff is an integer in the range of 0 to 32767,
inclusive. If sprop-max-don-diff is not present, the value of the
parameter is unspecified. sprop-max-don-diff is calculated as
follows:
sprop-max-don-diff = max{AbsDON(i) - AbsDON(j)}, for any i and
any j>i,
where i and j indicate the index of the NAL unit in the
transmission order and AbsDON denotes a decoding order number of
the NAL unit that does not wrap around to 0 after 65535. In other
words, AbsDON is calculated as follows: let m and n be consecutive
NAL units in transmission order. For the very first NAL unit in
transmission order (whose index is 0), AbsDON(0) = DON(0). For
other NAL units, AbsDON is calculated as follows:

If DON(m) == DON(n), AbsDON(n) = AbsDON(m)
If (DON(m) < DON(n) and DON(n) - DON(m) < 32768),
AbsDON(n) = AbsDON(m) + DON(n) - DON(m)
If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768),
AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)
If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768),
AbsDON(n) = AbsDON(m) - (DON(m) + 65536 - DON(n))
If (DON(m) > DON(n) and DON(m) - DON(n) < 32768),
AbsDON(n) = AbsDON(m) - (DON(m) - DON(n))
where DON(i) is the decoding order number of the NAL unit having
index i in the transmission order. The decoding order number is
specified in Section 5.5 of RFC 6184 [1].
Informative note: Receivers may use sprop-max-don-diff to
trigger which NAL units in the receiver buffer can be passed to
the decoder.
max-rcmd-nalu-size: This parameter MAY be used to signal the
capabilities of a receiver. The parameter MUST NOT be used for
any other purposes. The value of the parameter indicates the
largest NALU size in bytes that the receiver can handle
efficiently. The parameter value is a recommendation, not a
strict upper boundary. The sender MAY create larger NALUs but
must be aware that the handling of these may come at a higher cost
than NALUs conforming to the limitation.
The value of max-rcmd-nalu-size MUST be an integer in the range of
0 to 4294967295, inclusive. If this parameter is not specified,
no known limitation to the NALU size exists. Senders still have
to consider the MTU size available between the sender and the
receiver and SHOULD run MTU discovery for this purpose.
This parameter is motivated by, for example, an IP to H.223 video
telephony gateway, where NALUs smaller than the H.223 transport
data unit will be more efficient. A gateway may terminate IP;
thus, MTU discovery will normally not work beyond the gateway.
Informative note: Setting this parameter to a lower than
necessary value may have a negative impact.

sar-understood: This parameter MAY be used to indicate a receiver
capability and nothing else. The parameter indicates the maximum
value of aspect_ratio_idc (specified in H.264 [2]) smaller than
255 that the receiver understands. Table E-1 of H.264 [2]
specifies aspect_ratio_idc equal to 0 as "unspecified"; 1 to 16,
inclusive, as specific Sample Aspect Ratios (SARs); 17 to 254,
inclusive, as "reserved"; and 255 as the Extended SAR, for which
SAR width and SAR height are explicitly signaled. Therefore, a
receiver with a decoder according to H.264 [2] understands
aspect_ratio_idc in the range of 1 to 16, inclusive, and
aspect_ratio_idc equal to 255, in the sense that the receiver
knows exactly what the SAR is. For such a receiver, the value of
sar-understood is 16. In the future, if Table E-1 of H.264 [2] is
extended, e.g., such that the SAR for aspect_ratio_idc equal to 17
is specified, then for a receiver with a decoder that understands
the extension, the value of sar-understood is 17. For a receiver
with a decoder according to the 2003 version of H.264 [2], the
value of sar-understood is 13, as the minimum reserved
aspect_ratio_idc therein is 14.
When sar-understood is not present, the value MUST be inferred to
be equal to 13.
sar-supported: This parameter MAY be used to indicate a receiver
capability and nothing else. The value of this parameter is an
integer in the range of 1 to sar-understood, inclusive, equal to
255. The value of sar-supported equal to N smaller than 255
indicates that the receiver supports all the SARs corresponding to
H.264 aspect_ratio_idc values (see Table E-1 of H.264 [2]) in the
range from 1 to N, inclusive, without geometric distortion. The
value of sar-supported equal to 255 indicates that the receiver
supports all sample aspect ratios that are expressible using two
16-bit integer values as the numerator and denominator, i.e.,
those that are expressible using the H.264 aspect_ratio_idc value
of 255 (Extended_SAR, see Table E-1 of H.264 [2]), without
geometric distortion.
H.264-compliant encoders SHOULD NOT send an aspect_ratio_idc equal
to 0 or an aspect_ratio_idc larger than sar-understood and smaller
than 255. H.264-compliant encoders SHOULD send an
aspect_ratio_idc that the receiver is able to display without
geometrical distortion. However, H.264-compliant encoders MAY
choose to send pictures using any SAR.
Note that the actual sample aspect ratio or extended sample aspect
ratio, when present, of the stream is conveyed in the Video
Usability Information (VUI) part of the sequence parameter set.

Encoding considerations: This type is only defined for transfer via
RTP (RFC 3550) and is framed and binary (see Section 4.8 in RFC
4288).
Security considerations: See Section 9 of RFC 6185.
Interoperability considerations: None
Published specification: RFC 6185 and its reference section
Applications that use this media type: Video streaming and
conferencing applications
Additional information: None
Magic number(s):
File extension(s):
Macintosh file type code(s):
Person & email address to contact for further information:
Tom Kristensen <tom.kristensen@tandberg.com>, <tomkri@ifi.uio.no>
Intended usage: COMMON
Restrictions on usage: This type depends on RTP framing; hence, it
is only defined for transfer via RTP (see RFC 3550). Transport
within other framing protocols is not defined at this time.
Author: Tom Kristensen
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG
7. Mapping to SDP
The mapping of the above defined payload format media subtype and its
parameters SHALL be done according to Section 3 of RFC 4855 [10].
An example of the "fmtp" attribute in the media representation of a
level 2.2 bitstream is as follows:
a=fmtp:97 profile-level-id=008016

7.1. Offer/Answer Considerations
When H264-RCDO is offered over RTP using SDP in an Offer/Answer model
[5] for unicast and multicast usage, the limitations and rules
described in Section 8.2.2 of RFC 6184 [1] apply. Note that the
profile_idc byte of the H264-RCDO profile-level-id parameter can only
take the value of 0 (no profile).
For interoperability with systems not supporting H264-RCDO, it is
RECOMMENDED to offer the H264 media subtype as well. As specified in
RFC 3264 [5], listing the payload number for H264-RCDO before H264 in
the format list on the "m=" line signals that H264-RCDO is preferred
over H264. Following is an example where this scheme is applied:
m=video 5555 RTP/AVP 97 98
a=rtpmap:97 H264-RCDO/90000
a=fmtp:97 profile-level-id=008016;max-mbps=42000;max-smbps=323500
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=428016;max-mbps=35000;max-smbps=323500
7.2. Declarative SDP Considerations
When H264-RCDO over RTP is offered with SDP in a declarative style,
as in the Real Time Streaming Protocol (RTSP) [11] or the Session
Announcement Protocol (SAP) [12], the considerations in Section 8.2.3
of RFC 6184 [1] apply. Note that the profile_idc byte of the H264-
RCDO profile-level-id parameter can only take the value of 0 (no
profile).
8. IANA Considerations
IANA has registered H264-RCDO as specified in Section 6.1. The media
subtype has also been added to the IANA registry for "RTP Payload
Format MIME types" (http://www.iana.org).
9. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [6] and in any applicable RTP profile. Refer also to
the security considerations of the RTP Payload Format for H.264 Video
specification in RFC 6184 [1]. No additional security considerations
are introduced by this specification.